The use of fixation plates for the treatment of bone fusions and fixations has grown more prevalent over the past decade. Indeed, while early procedures using fixation plates were generally restricted to long bones and lower lumbar levels of the spine, fixation plates have increasingly found applications in other bone instrumentation such as in the cervical spine.
A typical bone fixation plate is provided with a plurality of bores therethrough. A corresponding plurality of fastener members, typically bone screws having a headed portion and a threaded shaft, are provided to secure the plate to the bone, or bones, to be fixated. A common problem with the use of fixation plates, regardless of their location, is the tendency of the bone screws to “back-out” of the underlying bone. This problem is particularly prevalent in areas of high stress such as the spine. Given the delicate nature of the spine, anything that may result in post-operative complications, such as plate movement or revision, can seriously endanger the patient's long-term prognosis.
Bone fixation systems have employed various techniques in an attempt to overcome the problem of screw back-out. Current techniques rely either on the use of specially designed bone screws, are irreversible, or require special procedures that could complicate the surgery. For example, U.S. Pat. No. 5,275,601 discloses a self-locking bone fixation system wherein the heads of the bone screws are frustoconical in shape and have a directionally corrugated outer surface; U.S. Pat. No. 5,269,784 discloses a threaded screw nut that threadingly engages a portion of the bone screw to thereby secure the bone screw to the fixation plate; U.S. Pat. No. 4,484,570 discloses a bone fixation system wherein the heads of the bone screws are hollow and expandable; and U.S. Pat. No. 5,578,034 discloses a bone fixation system in which the plates are heated after insertion, thereby expanding a retaining mechanism into place around the screw.
All of the cited prior art systems suffer from one or more undesirable drawbacks. First, some of these prior art systems rely on a retainer that itself uses a threaded connection to maintain the bone screws in position, meaning that the problem of screw back-out still exists. Second, several of these systems permanently seal the screw into place, rendering revision or alteration of the plate very difficult. Finally, the requirement that one use a particular, specially designed proprietary bone screw to prevent back-out limits a surgeon's ability to choose the best-engineered screw for a particular application because the proprietary bone screw may have inappropriate specifications such as thread pitch. Accordingly, a bone fixation system incorporating a mechanism for preventing screw back-out that is simple to use and revise and can be operated with any standard bone screw would be desirable.